This invention is in the field of sewage treatment equipment and is particularly directed to an aerobic sewage treatment unit having optimum aeration efficiency and being adaptable for converting an anaerobic sewage treatment unit to operate on the preferable aerobic process.
Septic systems employing the anaerobic process are widely used in areas not having public sewage disposal facilities. Generally, such systems consist of a sewage collection tank, usually referred to as a septic tank, in conjunction with a drain field into which effluent from the tank is discharged. Raw sewage normally flows by gravity into the septic tank where the solids gravitate to the bottom of the tank with liquid being discharged into the drain field to hopefully percolate downwardly into the surrounding soil.
The raw sewage received in septic tanks usually contain organic constituents including protein, carbohydrates and fats, oxygen activated organisms which function biochemically to decompose the organic constituents, and a limited natural supply of oxygen which activates the organisms to decompose the organic constituents. Initially in the biolysis of sewage deposited in the septic tank, urea, ammonia, and other products of the digestive putrefactive decomposition are partially oxidized so as to consume the limited amount of oxygen initially present in the raw sewage. Consequently, further decomposition of the sewage is by the anaerobic process. Continued putrefaction occurs under the anaerobic conditions so that the proteins are broken down to form urea, ammonia, foul-smelling mercaptans such as hydrogen sulfide and fatty and aromatic acids. Carbohydrates are broken down into their original fatty acid, water, carbon dioxide, hydrogen, methane and other substances. Fats and soaps are effected similarly to the hydrocarbon and are broken down to form the original acids of their constituency as well as carbon dioxide, hydrogen, methane and the like. Stable nitrides and nitrates are produced as the final product of the anaerobic decomposition process.
One substantial disadvantage of anaerobic sewage treatment in septic tanks is that there is an eventual accumulation of solid materials in the tank which must be periodically removed in order to avoid clogging of the drainfield and consequent discharge of raw sewage in the area of the tank. Additionally, the efficiency of a septic tank is largely dependent upon the soil conditions in which the drain field is located since the percolation of the soil is determinative of the size of the drain field and its consequent capacity for receiving and disposing of liquid effluent. In some areas, it is impossible to employ septic tanks due to the poor percolation characteristics of the soil.
The deficiencies of anaerobic process septic tanks have resulted in a substantial movement toward the usage of aerobic sewage treatment systems which also employ a receiving tank and a drain field but which additionally employ means for supplying oxygen to the sewage during its entire treatment process to provide a more complete decomposition than is normally obtainable in an anaerobic system. While the aerobic sewage treatment units that are commercially available have provided substantial advantages over the older anaerobic systems, they have suffered from a number of deficiencies from the standpoint of cost of manufacture, operation and maintenance which have slowed their acceptance as a substitute for the older anaerobic systems.
One of the more common problems with prior known commercial aerobic sewage treatment systems is that they fail to adequately dissipate the solid materials in the sewage and such materials consequently are discharged from the tank without full treatment. The discharge of solid materials into the drain field is highly undesirable since it can result in a blockage of the drain field within a relatively short time. In an effort to preclude the discharge of solid materials, some units have employed filters upstream of the outlet of the drain field. However, this approach has not proven to be satisfactory since the filters soon become clogged and must be replaced in order to remain effective.
Various other expedients have been employed in the prior known aerobic sewage treatment units such as aerators in the bottom of the main receiving tank, mechanical agitation devices, macerators and the like. Frequently, devices of the foregoing type have been overly complex and consequently prone to high expenses of initial manufacture as well as of maintenance. Other problems with prior known systems include failure to obtain optimum oxygenation in a rapid manner which necessitates the employment of a larger main tank than would be necessary if increased rates of oxygenation could be effected.
One example of a system proposed for incorporation into a septic tank system is disclosed in U.S. Pat. No. 3,662,890. Patentee proposes to withdraw liquid from the septic tank, treat it with air under pressure, and return the treated liquid to the septic tank. The patented system is completely hydraulic and relies on radially sparging of the air into the liquid. Pressure is maintained in the treatment vessel at all times by a gate valve in the discharge conduit.